1. Field of the Invention
This invention relates to a method for forming a thin film of a refractory metal, and more particularly to a method for forming a thin film of a refractory metal selectively on a substrate utilizing a vapor phase deposition technique.
2. Description of the Prior Art
Recently, semiconductor technologies have made great progress, and various attempts have been made to improve operational speed and integration rate of devices.
For instance, an attempt has been made in MOS integrated circuits to reduce the area of the contacting portions for electrical connection between elements and a wiring layer in order to cope with the miniaturization of elements in the integrated circuit. Further, to improve the operational speed, the depth of PN junction has been made smaller. The reduction of the contacting area and the depth of the PN junction, however, arise problems such as increasing the contact resistance and tending to break down the PN junctions during the formation of electrodes.
For instance, in a case where arsenic (As) ions are injected into a P type silicon substrate 1 shown in FIG. 3 to provide an N.sup.+ silicon diffusion layer 2 forming a PN junction and having a thickness of 0.1 micron, and an aluminum electrode 5 is provided for the N.sup.+ silicon layer 2 through a contact hole 4 provided through an insulating layer 3, there is a possibility of the PN junction being short-circuited by an interlayer reaction caused between silicon and aluminum respectively contained in the N.sup.+ silicon layer 2 and the aluminum electrode 5.
As a technique for preventing the interlayer reaction, a method has attracted attention in which a barrier metal between the aluminum electrode 5 and the N.sup.+ silicon layer 2 is interposed. According to this method, as a material for the barrier metal, titanium, tungsten, and the like are usually employed and the barrier metal is formed, for example, by depositing selectively on the N.sup.+ diffused layer 2 by use of a vapor phase growing method, the barrier metal may also be formed by depositing titanium silicide using the spattering method and then removing the deposition except the contacting portion by photo-lithoetching method.
In an actual fabrication, the N.sup.+ silicon layer 2 is formed in the P type silicon substrate 1 as shown in FIG. 4(a) by injecting arsenic (As) ions into the substrate 1 using silicon oxide film as a mask. A silicon oxide layer 3 is then formed on the entire surface of the substrate 1 and then N.sup.+ silicon layer 2. By the photo-etching method, a hole 6 of approximately 1.0 .mu..sup.2 is formed through the silicon oxide layer 3, and by a vapor phase deposition method, a thin layer 7 of, tungsten is selectively formed only at a portion defined by the hole 6 through which the surface of silicon is exposed outside as shown in FIG. 4(b), and then the aluminum electrode 5 is deposited on the tungsten thin layer 7 as shown in FIG. 4(c). In this manner, diffusion of silicon from the substrate into the aluminum electrode 5 during heat treatment can be prevented by the provision of the tungsten thin layer 7.
At this time, it is known that if the tungsten layer 7 is thick enough to bury the hold 6 completely and the aluminum electrode 5 is deposited after the surface of the tungsten layer 7 has been smoothed, the thickness of the aluminum electrode can be made even and an electrically reliable terminal connection can be thereby realized. However, it has not been easy heretofore to provide a diffused layer of such a large thickness by the selective vapor phase deposition method without losing the selectivity.
As for the formation of tungsten thin film using WF.sub.6 and H.sub.2, there are reports by Jerome J. Cuomo, "Selective Chemical Vapor Deposition of Tungsen", the third International Conference of Chemical Vapor Deposition, 1972, pp. 270-291, and by Constantin-Eugen Morosamu and Valerian Soltuz, "Kinetics and Properties of Chemically Vapor Deposited Tungsten Films on Silicon", Thin Solid Films, Vol. 52, pp. 181-194, 1978. However, the above described problem still remains unsolved despite of these reports. This problem commonly arises in the selective vapor phase deposition applied not only to the formation of tungsten film but also to the formation of any other refractory metals, and therefore thick layer of these metals has not yet been produced without losing the selectivity.